Abstract: A new optical technique is described that permits extension of cavity ring-down spectroscopy (CRDS) to surfaces, films, and liquids. As in conventional CRDS, the photon intensity decay time in a low loss optical cavity is utilized to probe optical absorption. Extension to condensed matter is achieved by employing intra-cavity total internal reflection (TIR) to generate an evanescent wave that is especially well suited for thin film chemical sensing. Tow general monolithic cavity designs are discussed: (1) a broadband, TIR-ring cavity that employs photon tunneling to excite and monitor cavity modes, and (2) a narrow bandwidth cavity that utilizes a combination of TIR and highly reflective coatings. Following a qualitative description of design features, a beam transfer matrix analysis is given which yields stability criteria and mode properties as a function of cavity length and mirror radius of curvature. A signal- to-noise ratio calculation is given to demonstrate the evaluation of sensitivity.!16
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